New insights into the piezoelectric, thermodynamic and thermoelectric properties of lead-free ferroelectric perovskite Na0.5Bi0.5TiO3 from Ab initio calculations

Ab initio DFT calculations have been performed to investigate a complete set of structural, electronic, elastic, piezoelectric, thermodynamic and thermoelectric properties of lead-free perovskite Na 0.5 Bi 0.5 TiO 3 (NBT) crystalizes in rhombohedral ( R 3 c ), tetragonal ( P 4 bm ) and cubic ( Pm 3 ̅ m ) phases. The lattice stabilities in Na 0.5 Bi 0.5 TiO 3 crystals are studied from the formation energy and phonon dispersions. Electronic band profiles obtained using new KTB mBJ+so potential reveal that R 3 c , P 4 bm and Pm 3 ̅ m phases of NBT are indirect band gap semiconductors with energy values of ~ 3.29, 3.05 and 3.09 eV, respectively. Analysis of the calculated elastic constants ( C ij ) indicates that NBT systems are elastically stable. Ferroelectric tetragonal NBT crystal shows relatively high piezoelectric coefficients [ d 15 = 101.0 pC/N, d 31 = 51.3 pC/N and d 33 = 81.1 pC/N], compared to the rhombohedral system [ d 15 = 96.4 pC/N, d 31 = 8.93 pC/N and d 33 = 21.2 pC/N and d 22 = 21.2 pC/N]. Thermodynamic quantities for rhombohedral NBT compound were calculated using the quasi-harmonic Debye model. Thermoelectricity in terms of electrical conductivity σ / τ , thermal conductivity κ / τ , Seebeck coefficient S , figure of merit ZT and thermo power for NBT crystals were examined using BoltzTraP2 code. P 4 bm -NBT structure showed a largest magnitude of S ~ 201.42 µV/K (at T = 500 K), following by Pm 3 ̅ m ~ 173.6 µV/K (800 K) and R 3 c ~ 158.6 µV/K (100 K). ZT maximized to high values ~ 2.76 (at 700 K), 2.16 (900 K) and 1.96 (400 K) for P 4 bm , Pm 3 ̅ m and R 3 c structures, respectively. Na 0.5 Bi 0.5 TiO 3 (NBT ) compounds could be a promising candidate for use in manufacturing high-performance piezoelectric devices and developing high-power thermoelectric generators. • Ab initio DFT calculations on Piezoelectric, thermodynamic and thermoelectric properties of perovskite Na 0.5 Bi 0.5 TiO 3 crystals. • NBT possesses higher hardness and better mechanical stability. • A indirect band gap character was obtained for the three considered phases. • NBT crystals have excellent thermoelectric and piezoelectric performances. • Lead-free NBT systems are promising candidate for developing high-power thermoelectric generators and piezoelectric devices.